Alternating current rectifier



March 15, 1932. R. H. MAXSON ALTERNATING CURRENT RECTIFIER Filed May 4,1925 ?atented (l5,

- attain -BOL a. .s mon- 1- ron, wrscoivsm; ASSIGNOB. '10 ran nmrcxconrona- TIQN, OF MTG]; WISCONSIN, A OOBIQBATION OF DELAWARE AL ATINGCURRENT RECTIFIER Application filed May 4,

This invention relates to alternating current rectifiers. It will beexplained as embodied in a rectifier particularly adapted for supplyinghigh- 5 pressure mercury arc lamps, such as are use in light therapy,withunidirectional current from an alternating current source.

Mercury arc lamps such as are used 1n light therapy for the productionof act1nic or ultraviolet light rays, ordinarily require a seriesballast resistance whose function is to limit the current flow duringthe starting period when the voltage drop across the lamp is relativelylow. After the vapor pressure in the lamp has increased and a cond1tionof equilibrium or stability is reached, this ballast resistance may beused to adjust the arc voltage. Heretotore rectifiers have been employedwhich utilize vacuum tube or electronic I valves, each of whichcomprises in general an evacuated or low ressure' inertgas filled vesselortube containing two electrodes, i. -e., an anode and a heated cathode.These valves, under proper operating condi- Lions, permit current toflow through them in (tune direction, but prevent the passage of currentin the reverse direction. Consequently, by a proper arrangement oftransformer windings and valves, both waves of the alterec, natingcurrent may berectified to produce a pulsating unidirectional-current.These current ulsations fall to zero twice for each alternatlng currentcycle, and since the arc lamp will not operate if the current fallsbelow a certain minimum value, it has been the practice to em lay areactance or inductive resistance in t tance, when the current isflowing to the lamp, causes the. current pulsations'to lag behind thevoltage pulsations, and the-current may be prevented from fallinfilbelow the critical value for the lamp. other words the react-ance servesto smooth but or straighten the unidirectional current waves.

To supply rectified or unidirectional current to a mercury arctherapeutic lamp at a typical voltage of approximately 7 5 volts and lamperes, it has been the practice to locate the ballast resistance inthe transformer 85 secondary or lamp circuit, that is, on the load elamp circuit. This reac-- 1925. Serial no. 27,673.

side of the transformer, as distinguished from its line side. In orderto overcome the voltage drop in the ballast resistance and reactance, itis necessary for the rectifier to ded liver about 120 D. C. volts inorder to provide the volts across the lamp terminals- The electronicvalve ordinarily employed will not operate satisfactorily when thevoltage across or between its electrodesis much in excess of 75 volts.Therefore it has been the practice to employ two valves in series torectify each half-Wave or cycle in order to obtain the desired 120volts. Thus, four valves have been required, and because the ballastresistance is located in the transformer secondary or lamp circuit, ithas been necessary with the arrangement heretofore used. to

make the transformer large enough to carry M the energy loss of'theballast resistance and cathode excitation in addition to its loadcurrent.

. One of the objects of this invention is to provide an improvedrectifier.

Another object is to provide an improved electronic valve type ofrectifier particular- 1y adapted for supplying mercury arc therapy lampswith unidirectional current from an alternating current source.

Another object is to provide a relatively inexpensive rectifier.

Another objectis to provide a rectifier requiring a smaller number ofvalves than have been necessary heretofore.

Another object is to provide a rectifier wherein the capacity or size ofthe transformer may be reduced.

Another object is to provide a rectifier which is eificient andrelatively simple.

' Other objects and advantages will hereinafter appear.

Anembodiment of the invention particularly adapted for use in lighttherapy is diagrammatically shown in the accompanying drawing.

In the drawing a typical air cooled highpressure mercury arc therapeuticlamp 5 is arranged to receive unidirectional current from an alternatingcurrent su ply line 6. The details of the lamp need not be describedbecause such lamps are well-known in the contacts with the anode, thuscompletin the circuit-and causing the current to start owing. Then thetube is tilted back. a ain to separate the mercury from the ano eand.

thus strike or draw the arc.

The improved rectifier has a transformer provided with a primary windingIOVand asecondary composed of two main sections 11 and 11' and twoexciting sections 12 and 12'. A variable resistance 13 is located in thetransformer primary circuit to maintain the voltage across the primarywinding at the roper value and to act as ballast resistance or the lamp.The inner ends of the transformer secondary sections 11 and 11' areconnected to one terminal of the lamp through a reactance 14. Thisreactance serves in the usual way to smooth out the waves of thepulsating rectified or unidirectional current and prevent the same fromfalling so low that the arc will go out or become unstable. The outerends of the main secondary sections 11 and 11 are connected to thecathode exciting or heating circuits of electronic valves 15 and 15',respectively. Each of these valves comprises an evacuated or lowpressure inert gas filled vessel-or tube containing two electrodes. Theoperation of the valve is dependent upon the heating of the cathode ornegative electrode. The cathode 16 of valve. 15 is heated or excited bycurrent from exciting secondary 12, and the cathode 16' of valve 15' isexcited or heated by current from exciting secondary 12'. The anodes orpositive electrodes 17 and 17 of valves 15 and 15', respectively, areconnected to the cathode terminals of the mercury arc lamp.

. For the production of 4% am res of cur-- rent at a normal rating votage of approximately 75 D. are lamp terminals from a line voltage. of110 volts A. (3., good results have been obtained by winding thetransformer primary with 87 turns of #18 B. & S. gauge wire, and each ofthe main secondary sections with 112 turns of #15 B. & S. gauge wire.Thus, when the bal ast resistance is adjustedto provide about 85 voltsacross the primary, the voltage across the two main sections of thesecondary will be about 219 :volts A. C. or a rectified voltage of about75 volts. Each of the cathodeexciting sections of the secondary is.

wound with two turns of B. & S. gauge wire which will provide a maximumof about Woks across each cathode at no; load.

e operation of the rectifier is as follows:

The alternating voltage impressed upon the primary windin induces analternatinglpotential across a of the sections or 'ngs alternatingcurrent directional current, t e frequency being twice' volts at themercury nuance qo f the secondary. Since the excitation of .v the valvecathodes is dependent entirely upon the heati efiect of the currentflowi therethrou h, t e current induced in exciting secondaries I2 and12'.is not rectified but flows through the respective cathode filamentsas alternating current. The heating of the cathodes renders the valvesoperative to permit current to flow therethrough from the anode to thecathode, although current is still prevented from flowing in theopposite direction as long as the voltage impressed across theelectrodes does not exceed their break-down value, which ordinarily isabout 75 volts. Consequently, during one-half wave of the alternatinvoltage induced in the main seconary sections 11 and 11', current willflow, for example, from the common point between the main secondarysections throu h the reactance 14 and lamp 5 and throug valve from itsanode 17 to its cathode 16, and thence to the outer end of secondarysection 11. During this half-wave, however, valve 15' prevents currentfrom flowin in the opposite direction. On the next hal -wave, currentflows from the point between the main secondary sections throughreactance 14 and lamp 5 and through valve 15' from its anode 17' to itscathode 16 and thence to the outer end of secondary section 11'. Valve15 pmvents the flow of current in the opposite direction during this.half-wave. Thus the rgovides a pulsating unithe frequency of t ealternating current. The volta e across the electrodes of the electronicva ves during all load conditions is suficiently limited to preventinjury thereto or interfere with their valve action. When i the 'arcisnot burning, however, and cons'equently the only current flowing in thepri, mary circuit is that drawn by the exciting secondaries, the voltageacross the valve electrodes mag be sufliciently high to'cause thevalvesto reak down or pass current in the wrong direction. It has beendiscovered that this breaking down under no load conditions canbeeliminated by putting a small load on the valves. A voltmeter Vconnected across the, leads to the mercury arc lamp has been found toafford sufiicient load under ordinary conditions. I

The improved rectifier permits the elimination of two of-the valveswhich are essential to the prior rectifiers andyet the voltage impressedacross their electrodes maybe kept at safe values. Since the ballastresistance is located onthe line side instead of the lamp or load sideof the transformer, the transformer does not have'tosupply the energylost in their resistance during operation. Al-

so the current required to excite the valve cathodes is reducedbecausethe number of valves is reduced. Consequently, the capaci ty andsize of thetransformer may be matearc lamp and each permitting onlyalternate half-v current are needed. However,

rially reduced. It has been found that for the ordinary therapeuticmercury arc lamp the size of the transformer may be reduced in theneighborhood of 35%. Another feature of advantage is that the maximumvoltage is impressed upon the valve cathodes or filaments before theload, i. e., the lamp, is turned on, but drops to a relatively low valueas soon as current is flowing trodes. Thus the maximum voltage isimpressed at a time when the to a relatively low value after the currentbetween the electrodes is flowing and when only a relatively lowexciting current is required.

Although a rectifier employing only a pair of valves has been describedin detail, it will be understood that a greater number may be used whenhigher voltages of unidirectional the ratiobetween the number of valvesused in my rectifier and the rectifier heretofore used will remain thesame. That is for a given voltage my rectifier will require only half asmany valves as the prior rectifiers.

I claim: I g I l. The combination of a mercury arc vacuum tube having arelatively low resistance at starting and a high resistance whennormally operating, a valve for rectifying each half-Wave of analternating current source, a transformer and ballast resistance in theprimary circuit thereof for limiting the 'alternating voltage impressedfrom the source upon the valves and automatically varying the voltageimpressed on the tube as the resistance thereof changes, and a reactanceinterposed between the tube and the transformer to lessen the tube.

2. In combination a high pressure mercury having an anode and a cathode,a pair of valves connected to the lamp cathode and each permitting. onlyalternate halfwaves of alternating current to pass therethrough to thelamp, a transformer having a primary winding for connection to analternating current source and" a secondary winding which is connectedat a different point to each valve to the lamp anode, the transformerservin to reducethe voltage of the induced hal waves to a valve withinthe rectifying range of the valves, and'a ballast resistance in thecircuit between the transformer primary and the source. v

3. In combination a high pressure mercury arc lamp having an anode and acathode,

a pair of valves connected to the lamp cathode waves of alternatingcurrent to pass therethrough to the lamp, a transformer having a primarywinding for connection to an altprnating current source and a secondarybetween the valve elec-- valve is supposed. to pick up or beingrectifying, but falls.

the amount of the fluctua-' 'tions of unidirectional current impressedon and at a common point waves to a value within the safe rectifyingrange of the valves, a ballast resistance in the circuit between thetransformer primary and the source, and means for preventing thebreaking down of the valves when the same are subjected to voltage whilethe lamp is extinguished.

4. In combination a high pressure mercury arc lamp having an anode and acathode, a pair of valves connected to the lamp cathode and eachermittingonly alternate half- Waves of a ternating current topass'therethrough to the lamp, atran'sformer having a primary windingfor connection to an alternating current source and a secondary windingwhich is point to each valve and at a common point to the lamp anode,the transformer acting to reduce the voltage of the induced half-wavesto a value within'the safe rectifying range of the valves, a ballastresistance in the circuit between the transformer primary and thesource, and a relatively small load permanently in circuit'between thevalves and the lamp to prevent excessive "voltage being applied to thevalves when the lamp is not in service.

In witness whereof I ROLLAND H. MAXSON.

connected at a different hereunto subscribe my name this 27th day of"April, 1925'.

